test the Numitrons.
Note also the diode D2 and jumper JP2 (Figure 12a).
They were intended to be a rudimentary option for
lowering the brightness of the Numitrons and lower the
current requirement slightly. It simply relied on the forward
voltage drop of the diode, with a jumper to short it
(Figure 12b). It is not really required and may be left off
the board, in which case a simple jumper wire may be
inserted in place of the diode.
The clock is set using only two buttons with
multiple functions to control 12/24 hour display
selection, LED pattern selection, as well as
setting the time.
difficult things to
read in high
circuits since the
in a switch or
bounce like a
dropped ball; it
takes a bit of time
to stop settling in one state or the other when pushed.
Debouncing can be done in software, but in this case, a
hardware solution was used. The solution is based on the
RC time required to charge/discharge a small capacitor
When the button closes, the capacitor (C7) is
discharged through a resistor (R19), allowing the bounce
to settle before the voltage on the PIC input pin falls
below the logic level 0. When released, the capacitor is
charged quickly through R18 and D4. By using this
method, no further precautions are required in the
software, and the button is treated like an ideal button
September 2016 31
■ FIGURE 12A.
Location of the
diode and jumper
JP2 on the PCB.
■ FIGURE 12B.
location of the
■ FIGURE 11A.
Close-up of the
CD4511 BCD to
■ FIGURE 11B.
snugly atop the